Tom Standage is the digital editor at The Economist and the author of the forthcoming book “Writing on the Wall: Social Media — The First 2,000 Years.”LONDON — SOCIAL networks stand accused of being enemies of productivity. According to one popular (if questionable) infographic circulating online, the use of Facebook, Twitter and other such sites at work costs the American economy $650 billion each year. Our attention spans are atrophying, our test scores declining, all because of these “weapons of mass distraction.”Yet such worries have arisen before. In England in the late 1600s, very similar concerns were expressed about another new media-sharing environment, the allure of which seemed to be undermining young people’s ability to concentrate on their studies or their work: the coffeehouse. It was the social-networking site of its day.